docs: add SECURITY.md (#9062)

* docs: add SECURITY.md

This moves existing security policy from README to SECURITY.md
which has a special meaning on Github:
https://docs.github.com/en/code-security/getting-started/adding-a-security-policy-to-your-repository



* style: editoral fix

Co-authored-by: Steve Loeppky <biglep@protocol.ai>